1. Field of the Invention
The present invention relates to an apparatus for adjusting a focal point of a zoom lens. More precisely, it relates to a focus adjusting apparatus having a detecting device which detects an angular position of a cam ring.
2. Description of Related Art
In a conventional zoom lens barrel, a code plate and a brush unit which come into electrical contact are provided between a cam ring, which is rotated to vary a focal length, and an immovable portion of the lens barrel to detect the focal length of the zoom lens.
Upon assembly of a zoom lens, it is necessary to adjust a back focal point (i.e., make a back focus adjustment) to make the focal point coincident with a film plane. The back focus adjustment is carried out by the movement of the entire lens system in the optical axis direction after the zoom adjustment is completed, so that no change of the focal point occurs during the zooming operation.
However, the back focus adjustment causes the position of the cam ring in the optical axis direction to change, thus resulting in a change in the relative position of the code plate and the brush unit in the optical axis direction. This problem can be solved by increasing the width of the conductive land on the code plate so as to compensate for the change in the relative position of the code plate and the brush unit caused by the back focus adjustment. However, this prevents a miniaturization of a compact camera.